Progress in the production of integrated circuits employing large scale integration and high operating speeds has been characterized by an ever decreasing pattern rule. It was expected in the “National Technology Roadmap for Semiconductors” (Semiconductor Industry Association, 1994) that the mass scale production of 0.18-μm rule devices would start from 2001. Actually, the production started in 1999, two years earlier than expected. For the production of 0.18-μm devices, ArF lithography was believed promising, but KrF lithography has been prolonged. KrF lithography is contemplated for the mass-scale production of 0.15-μm devices now and even 0.13-μm devices under future consideration. The mature KrF lithography accelerates microfabrication toward an ever decreasing feature size.
In these years, engineers also place a focus on the mask production technology. From the era of g-line and i-line reducing projection stepper have been used, with their reduction ratio of 1/5. Recently, a reduction ratio of 1/4 is employed in conjunction with the enlargement of chip size and the increasing aperture of projection lens. Not only a reduction in line width as a result of progress of micropatterning, but also a reduction in line width as a result of magnification change pose a serious problem to the mask production technology.
The exposure system for use in mask production has been changed from a laser beam exposure system to an electron beam exposure system in order to increase the line width accuracy. Since definition of a finer feature size becomes possible by increasing the acceleration voltage in an electron gun in the system, the acceleration voltage has increased from 10 keV to 30 keV. An acceleration voltage of 50 keV now becomes mainstream.
In conjunction with the increasing acceleration voltage, a lowering of resist sensitivity becomes a problem. As the acceleration voltage increases, the influence of forward scattering within the resist film is reduced so that the contrast of electron writing energy is improved, resulting in improvements in resolution and dimensional control. However, since such electrons can penetrate straight through the resist film, the sensitivity of the resist lowers. Since the mask exposure system carries out exposure by direct writing along a continuous stroke, the lowering of resist sensitivity undesirably leads to a lowering of productivity.
To meet the demand for higher sensitivity, chemical amplification type resists are now under consideration. However, there arises a problem that the sensitivity varies during vacuum standing after exposure. In particular, positive resists suffer from the problem of a footing profile on chromium substrates.
When resist film is allowed to stand in vacuum for a long time after exposure, the acid generated by exposure evaporates. If the acid evaporates before progress of elimination reaction, the resist becomes low sensitive. Even when post-exposure baking (PEB) is effected in dry air, nitrogen or argon, the heat treatment before progress of elimination reaction causes the acid to evaporate, resulting in a lower sensitivity.
It is believed that footing on substrates is the same phenomenon as the problem that KrF lithography encounters on basic substrates such as TiN. It is effective for alleviating the footing phenomenon to use acetal and other acid labile substituent groups having low activation energy in elimination reaction. For resists using acetal substituents, however, a phenomenon that sensitivity gradually lowers during vacuum standing after exposure was observed. In the presence of moisture, acetal groups undergo acid-catalyzed elimination reaction in a substantially irreversible manner. In the absence of moisture, however, the elimination reaction becomes reversible. Since the rate of reverse reaction is high, the apparent rate of elimination reaction is very low.
One effective means for preventing irreversible reaction of acetal in the absence of moisture is by adding high-boiling alcohols as disclosed in Hatakeyama et al, JP-A 11-15163. High-boiling alcohol is left within the resist film after pre-baking or during vacuum standing and functions to induce irreversible elimination reaction as does water. However, if the elimination reaction in vacuum becomes irreversible by the addition of alcohol, the elimination reaction proceeds during vacuum standing, resulting in a higher sensitivity. A mere combination of acetal groups with alcohol additives was difficult to enhance the stability in vacuum.